[20181015]为什么是3秒.txt

[20181015]为什么是3秒.txt

--//以前测试:连接http://blog.itpub.net/267265/viewspace-2144765/=>为什么是12秒.txt.
--//很奇怪12.1.0.1版本测试12秒(windows版本),而11g是3秒(在使用标量子查询的情况下).不知道为什么?
--//在12.2.0.1下测试看看:

1.环境:
scott@test01p> @ver1
port_string                    version        banner                                                                               con_id
------------------------------ -------------- -------------------------------------------------------------------------------- ----------
ibmpc/win_nt64-9.1.0           12.2.0.1.0     oracle database 12c enterprise edition release 12.2.0.1.0 - 64bit production              0

sys@test01p> grant execute on  dbms_lock to scott;
grant succeeded.

2.建立函数:
create or replace function get_dept (p_deptno dept.deptno%type)
   return dept.dname%type
is
   l_dname   dept.dname%type;
begin
   dbms_lock.sleep (1);

   select dname
     into l_dname
     from dept
    where deptno = p_deptno;

   return l_dname;
end;
/

3.测试:
scott@test01p> set timing on
scott@test01p> set feedback only
scott@test01p> select empno, ename, deptno, get_dept(deptno) c20 from emp;
     empno ename          deptno c20
---------- ---------- ---------- --------------------

14 rows selected.
elapsed: 00:00:14.00
--//14秒,这是正确的,14条记录.调用14次需要14秒.

--//换成标量子查询:
scott@test01p> select empno, ename, deptno, (select get_dept(deptno) from dual )c20 from emp;
     empno ename          deptno c20
---------- ---------- ---------- --------------------

14 rows selected.
elapsed: 00:00:03.03

--//执行时间是3秒,这次是正确的,因为标量子查询缓存结果,而仅仅有3个部门在emp表.这样3秒就正确了.

4.继续探究:
scott@test01p> set timing off
scott@test01p> alter session set statistics_level=all;

session altered.

scott@test01p> select empno, ename, deptno, (select get_dept(deptno) from dual )c20 from emp;
     empno ename          deptno c20
---------- ---------- ---------- --------------------

14 rows selected.

scott@test01p> set feedback on
scott@test01p> @ dpc '' ''
plan_table_output
-------------------------------------
sql_id  apagxtf1p2puy, child number 1
-------------------------------------
select empno, ename, deptno, (select get_dept(deptno) from dual )c20
from emp
plan hash value: 1340320406
--------------------------------------------------------------------------------------------------------------------
| id  | operation         | name | starts | e-rows |e-bytes| cost (%cpu)| e-time   | a-rows |   a-time   | buffers |
--------------------------------------------------------------------------------------------------------------------
|   0 | select statement  |      |      1 |        |       |     9 (100)|          |     14 |00:00:00.01 |       8 |
|   1 |  fast dual        |      |      3 |      1 |       |     2   (0)| 00:00:01 |      3 |00:00:00.01 |       0 |
|   2 |  table access full| emp  |      1 |     14 |   182 |     3   (0)| 00:00:01 |     14 |00:00:00.01 |       8 |
--------------------------------------------------------------------------------------------------------------------
query block name / object alias (identified by operation id):
-------------------------------------------------------------
   1 - sel$2 / dual@sel$2
   2 - sel$1 / emp@sel$1
--//从执行计划也可以发现fast dual执行了3.再次说明12.1版本有问题.
--//也再次说明oracle任何xx.1版本不能在生产系统使用.

5.继续测试使用 deterministic functions:
--//一般如果在在某个函数定义索引，需要deterministic,表示返回结果固定。其实即使不固定，也可以这样定义。
create or replace function get_dept (p_deptno dept.deptno%type)
   return dept.dname%type
   deterministic
is
   l_dname   dept.dname%type;
begin
   dbms_lock.sleep (1);

   select dname
     into l_dname
     from dept
    where deptno = p_deptno;

   return l_dname;
end;
/
scott@test01p> show array
arraysize 200
--//arraysize=200

scott@test01p> set timing on
scott@test01p> select empno, ename, deptno, get_dept(deptno) c20  from emp;
...
14 rows selected.
elapsed: 00:00:04.06
--//这次正确了4秒.大家可以自行设置array=2等各种情况.
--//为什么?大家可以看看我写的.http://blog.itpub.net/267265/viewspace-2138042/=>[20170426]为什么是4秒.txt

6.最后补充测试result cache的情况:

create or replace function get_dept (p_deptno dept.deptno%type)
   return dept.dname%type
   result_cache
is
   l_dname   dept.dname%type;
begin
   dbms_lock.sleep (1);

   select dname
     into l_dname
     from dept
    where deptno = p_deptno;

   return l_dname;
end;
/

scott@test01p> select empno, ename, deptno, get_dept(deptno) c20  from emp;
14 rows selected.
elapsed: 00:00:03.07

scott@test01p> select empno, ename, deptno, get_dept(deptno) c20  from emp;
elapsed: 00:00:00.07

--//第1次执行3秒,第2次执行0秒,因为结果缓存了.第二次执行直接取结果.修改如下结果一样.
create or replace function get_dept (p_deptno dept.deptno%type)
   return dept.dname%type
   result_cache
   deterministic
is
   l_dname   dept.dname%type;
begin
   dbms_lock.sleep (1);

   select dname
     into l_dname
     from dept
    where deptno = p_deptno;

   return l_dname;
end;
/

总结:
--//再次验证我以前的结论oracle 任何xx.1版本不要在生产系统使用.